1. Field of the invention
The present invention relates to a braking force control device which controls braking force applied to vehicle wheels in order to restrain the slip of the wheels.
2. Description of the Related Art
This kind of braking force control device controls the braking force applied to the wheels to restrain the wheel deceleration slip (lock), and is well-known as an anti-skid control system or an anti-lock braking system (hereinafter referred to as the ABS unit). In such an ABS unit, the wheel speed decreases as a driver actuates a brake pedal, and when the wheel slip becomes an ABS-control level (or, pressure decreasing level), the brake oil pressure decreases to prevent the wheel slip, and increases again after the wheel speed increases.
On the other hand, the above ABS unit controls the braking force according to the coefficient .mu.B of the braking friction (hereinafter referred to as the .mu.B characteristic) shown in FIGS. 17A and 17B. In other words, a range of slip ratio under Sa (optimum slip ratio) which corresponds to a peak value of the .mu.B characteristic (hereinafter referred to as .mu.B peak) is a relatively stable range and a range above Sa is unstable and may cause the wheel locking. In the stable range, the braking friction increases as slip increases with high lateral-holding force. In the unstable range, the braking operation becomes unstable because of the slip in excess of Sa. If the brake oil pressure is decreased before the slip ratio becomes Sa in ABS control, braking force may not increase sufficiently, thereby causing a long stopping distance. Therefore, the slip ratio to start the ABS control is set slightly above Sa in order to shorten the stopping distance.
However, the above conventional ABS unit can not achieve the ABS control at the ideal slip ratio. That is, the .mu.B characteristic (.mu.B peak) shown in FIGS. 17A and 17B and the Sa change as the road surface condition changes, and the range for the maximum braking force (slightly above Sa) also changes with the road surface condition. Accordingly, there is a possibility that an actual pressure decreasing slip ratio Sb shifts to the unstable range considerably from the optimum slip ratio Sa as shown in FIG. 17B. In this case, the pressure starts decreasing after an optimum timing, thereby causing unstable running of the vehicle and excessive pressure decrease due to abnormal slip.